Date Format!

Hi!

I am facing a problem on date format.  I want to know is there is any way to
add days to a date.
Currently, I retrieve the date value from the XML using substring.
But that is not recognized as a date format.
Therefore, when I add a value to it, say for example 3 + $deliveryDate, it
won't be able to produce the exact result.
I saw a lot of article written about the xalan or exslt in processiong the
date format.
However, I would like to know whether there is any general way to implement
it.

Please help.
Your help is really appreciated!